This invention relates to the field of optical data transmission systems, e.g., fiber optic communications, and specifically to variable optical attenuators for use in such systems.
The instrumentation disclosed in this application was developed to function as a testing device, i.e., as a diagnostic tool, but it may also be used as a permanent portion of a fiber optic system.
Between the transmitter and receiver of a fiber optic system, attenuation of the signal strength occurs. The system is designed for a normal amount of signal loss between transmitter and receiver. At the receiver, the signal strength must be maintained within an appropriate range between overly strong and unduly weak. The minimum strength is determined by the need for an adequate signal-to-noise ratio. The maximum strength is determined by the need to avoid flooding the receiver with excessive radiation. In other words, problems are created by either under-driving or over-driving the receiver.
The ideal fiber optic transmission system has two primary capabilities. It can handle very high frequencies of multiplexed signals. A current goal is to have a bit rate of over 1 gigabit per second. The other necessary capability is a low bit error rate.
In a given system, the desired high bit frequency rate and low bit error rate will be obtained in a certain range of attenuation, measured in decibels at the receiver. In other words, as stated above, problems are encountered if the attenuation dB goes above a certain value or below a certain value. The points at which an excessive bit error rate occurs are measured both to determine the desirable dB range when the optical transmission system is installed, and to diagnose problems which occur during subsequent operation of the system. Bit error rate (BER) testing provides (a) a measure of system performance in terms of the number of errored bits per transmitted bits, and (b) a measure of sensitivity in terms of minimum received optical power required to achieve a given (low) BER. An error detector in the system is required to detect a BER which is too high for satisfactory operation.
A variable optical attenuator (VOA) is primarily a testing tool for diagnosing BER problems. A VOA is an opto-mechanical device capable of producing a desired reduction (i.e., attenuation) in the strength of a signal transmitted via fiber optic cable. The VOA should produce a continuously variable signal attenuation from 0 dB to 70 dB, while introducing a normal insertion loss of only 1.5 to 3 dB, and exhibiting an optical return loss greater than 20 dB. If the attenuator causes excessive reflectance back toward the transmitter, its purpose will be defeated.
The traditional mechanism used as a VOA is a continuously variable neutral density filter. This is essentially an optical glass substrate with a metallic coating. The density of this coating increases as it rogresses across the surface of the filter. Attenuation is created by absorption and Fresnel reflection as the filter is moved relative to the optical beam. Generally, the movement of such a filter is rotation around the axis of the attenuated beam. VOAs of the variable density filter type are expensive devices which also exhibit some weakness in spectral absorption, reflection, and refraction characteristics. A less expensive device which eliminates certain flaws of such filters is highly desirable.
A much less expensive, but crude, VOA is a full light blocking member which is moved gradually across the optical beam in a direction perpendicular to the beam. Such a blocker may be provided by a threaded element whose upper edge acts as a knife edge in cutting across the optical beam. Although these devices are inexpensive, they have very poor sensitivity. In other words, they are not capable of the fine resolution in dB measurement which is required in the critical portion of the attenuation curve.